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Is there any benefit of using a swept wings at low-subsonic speeds?
Is there any benefit to using swept wings for speeds up to 250km/h ?
(By the way..Why do some fast birds like falcons, swifts and most migratory birds use swept wings or raked wingtips since they fly at low subsonic speeds?)
Peregrine falcon - swept wings

Alpine swift - swept wings

Albatros - straight wings with raked wingtips

Common swift - straight wings with raked wingtips

Don't all birds fly at low subsonic speeds, even the really fast ones?

Subsonic is every speed below Mach 0.8 (987km/h), birds belong in this category.Fastest bird in horizontal flapping flight is swift (170km/h) and peregrine falcon in dive (390km/h)

Note that birds have variable geometry wings, and all—including swallows, swifts and falcons—usually fly with straight wings when their intent is best glide range. They sweep the wings for higher speed, but it is more because of which adjustments they can make than a goal of its own.

@JanHudec Swept wing has lower drag coefficient compare to straight wings?Is this reason why birds sweep wing for high speed?

@member2017, by partly folding the wing the bird changes some parasite drag, which increases with speed, for some induced drag, which decreases, moving the optimal point to higher speed. The drag is higher at this point than with fully stretched wing at its lower optimal speed, but lower than keeping the wing stretched at the higher speed.

@JanHudec Two test with same wing  in wind tunnel, first test straight wing,second test with sweep(give backaward angle to that wing),which will show less drag force?

@member2017, it would depend on the flow speed. For each wing there is a speed at which it produces least drag (assuming we want it to produce constant lift) from which the drag increases both when speeding up and slowing down. And reducing the aspect ratio increases that speed, so sweeping the wing, which reduces aspect ratio, reduces drag at the high end of the speed range only.

@JanHudec Are you talking about reducing drag at high speed close to mach 1 or?Because  I mean only for low speed up to 250km/h...

@member2017, no, I am talking about reducing drag between any two speeds. The sweep is just a side-effect. The change in aspect ratio is what matters. Optimal wing for the higher speed would be just stubbier, but still straight. But birds can't change the length of their feathers, so they can't make their wings stubbier. But they can bend their ‘forearms’ a bit forward and the ‘fingers’ a bit back, which gives similar benefit with the side-effect of sweep.

@JanHudec thanks, I have here specific topic about sweep and drag at low-subsonic,so you can give your contribution if you want...https://aviation.stackexchange.com/questions/80668/can-wing-sweep-decrease-drag-force-at-low-subsonic-speeds

Birds adjust wing area by pulling in their humerus and ulna so the wing sweeps first forward, and to compensate for that, have to sweep their outer wing backwards which is accomplished by folding the digits backwards. Instead of adjusting lift coefficient, they adjust wing area to not expose more surface to air friction than necessary.
The picture below shows how the wing is opened for maximum lift at low speed. As Zeiss Ikon correctly observes, folding the digits back and fore is the bird's way of pitch control, so the sweep is a consequence of pitch control, not aerodynamics.

Red-tailed Hawk, captive bird, Bacara, Santa Barbara, California. By Steve Jurvetson / CC BY (https://creativecommons.org/licenses/by/2.0)

Does swept wing has lower Cd than straight wing?Every bird at high speed put wing back,isnt this reducing drag force?

@member2017 It certainly has a lower wing area. Drag coefficient might be the same or a bit lower due to the higher Reynolds number, but it is the product of area, dynamic pressure and drag coefficient that counts. And this product is certainly lower with a smaller wing area, so yes, pulling the outer wing back because of a pulled-in inner wing  is reducing the drag force.

Birds can also pull outer wing back when keep inner wing straight,(albatros at my post).All pointed wings birds  has zero lift at wingtips-bell lift distribution.

in my topic you say that sweep primary feathers dont have nothing to do with aerodynmics,but in this threads you explain raked wingtips with birds wings.can you explain why?.https://aviation.stackexchange.com/questions/8544/is-wingtip-design-mature/8554#8554    https://aviation.stackexchange.com/questions/19073/how-does-the-raked-wingtip-of-the-boeing-787-work/19078#19078

@member2017 Raked wingtips work best when restricted to wingtips and not when they constitute most of the wing. Birds use them, too, but when they pull in the inner wing the result does not make a "raked wing".

Swept wings may be useful to adjust the center of lift when, for instance, a change in power plant or other equipment has resulted in an aft center of mass location, or when it's desirable to allow additional after cargo storage.  They can also provide roll stability similarly to dihedral when flying at significant angle of attack.
Birds that have wing sweep generally seem to have the ability to vary this condition in flight -- which they appear (to my eye -- I'm not an ornithologist or even a bird-watcher) to use similarly to managing center of lift as above.  In fact, almost all birds do this to some extent, along with tail movements, for pitch control.

When you say "additional after cargo storage", do you mean like, an unlucky chipmunk now gripped in talons?

@Harper-ReinstateMonica Please tell me you're capable of at least distinguishing a shift from airplanes to birds in the discussion...

I was shown a hang glider with mild sweep to the wings, and a few degrees of tip washout, that is, the angle of attack was slightly lower at the wingtips (maintained by tensioning the rigging, I think).
The idea of the washout was that the central part of the wing stalled while the wingtips (with lower AOA) were still flying.
And the idea of the sweep was that the wingtips being aft of the centre section, the centre of lift moved aft when that happened, allowing the nose to drop, automatically recovering from the stall.
So, sweep can be used to help stability, and I'm pretty sure that hang glider stayed well below the transonic region.
EDIT : Tip washout and mild sweep visible here around 0:40 and 1:20, and clearly low Mach number.

I'm pretty sure that hang glider stayed well below the transonic region
Citation needed? :P

Swept wings increase something known as the critical mach number. When air travels over the top half of an airfoil, it gains speed. If the incoming air far upstream from the wing is at a high subsonic speed, the speed gained as it travels over the wing can induce transonic, sonic, or even supersonic flow which can lead to unpredictable wing effects such as control reversal and loss of control as the P-38 pilots of WWII faced.
Swept back wings increase the amount of span-wise flow, decreasing the amount of velocity that the air picks up as it travels over the airfoil and allowing the wing to fly closer to the speed of sound.
Also check out supercritical airfoils which delay mach effects as well.
Hope this helps!

Given the description of the question, you may precise your answer to focus more on low-subsonic speeds.


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Can hang glider hover straight into wind and keep same position?
Isnt resultant force allways  pointing slighlty backward at every wing,so why hang glider is not push downwind by the wind?
From theory that cant be possible,so does wind somehow coming at him at angle from below? ground seems flat,maybe slightly angled..
(Is it easy what his doing or that can do only very very skilled pilots?)
Can he penetrate continusly forward into wind direction in zag-zag motion,first straigt up and then down-forward or maybe with dynamic soaring like birds do?

Important to note that the hang glider isn't actually hovering.  It's flying like it always does, just not fast enough to move forward.  A hover means no forward airspeed.

@JohnK He cant  move continously forward,like sailboat does?Can he going from that beach to 500m out at sea?

Does this answer your question? Is it possible to fly backward if you have really strong headwind?

He's flying into a headwind that is the same velocity as his forward  airspeed.  Therefore, ground speed = 0.  Yes he can move forward by increasing his airspeed by lowering the nose of the glider to descend more steeply and faster.  But in that particlular upslope wind angle and velocity, this may increase his descent rate which results in closing with the surface, so he can only do it if he has clearance below to allow for the steeper descent, or if the upslope wind velocity increases.

@JohnK: You're absolutely right about almost everything you said. However, a hover does not mean no forward airspeed. A hover means means holding a fixed position relative to a stationary spot on the ground. When I hold hover in a helicopter with a headwind, it is with a certain degree of forward cyclic - the forward airspeed is not zero. Unless it is a windless day, a hover will always involve horizontal airspeed in some direction.

@AaronHolmes yeah ok fair enough.

"Wind" means something completely different to aircraft than what it means to those of us who are standing on the ground. Once you're in the air, "wind" is just the difference between your airspeed and your speed over the ground. I had some flight training in a little Cessna (never went for the license) and I remember one day, while practicing slow flight, I noticed that I was stationary over one spot on the ground. But that wasn't "hovering." I was flying at 40 knots through an air mass that coincidentally happened to be moving at 40 knots in the opposite direction relative to the ground.

Why is this question closed as a duplicate? The linked question is about ground speed vs air speed, not about ridge lift.

The ground is not flat, and indeed slightly angled. This is enough to "hover".
Specifically, the ground slope should be equal or more than the max L/D ratio of the hang glider at the current wind speed. The wind should go 'up' as much as the glider goes 'down' per unit of distance. The ground seems to go up about 2m over a distance of maybe 10m, which is a slope of one in five. An L/D of 5 seems perfectly reasonable for a hang glider.

Yes, the force from the wing is indeed always pulling slightly backwards. Upwards and slightly backwards relative to the wind that is. Since the wind here is coming in slightly upwards along the slope of the beach instead of horizontally, the force from the wing is turned to clean upwards instead of slightly backwards.
Here is the link to a force diagram showing a gliding plane in another thread. Just change the glide velocity vector "V" in the picture with the opposite of the wind vector on the beach in the film.
Edit: Presumably, this exercise is difficult and dangerous as it takes place very close to the ground. However, the so called "ground effect" might add a bit of stability. The closer to the ground, the less induced drag a wing experiences; hence there is relatively speaking more lift the lower you go, or relatively less force to sweep you downwind.
Edit 2, on risks: After learning from another answer that the practice of hovering close to the beach is called "dune gooning", i googled it and it really seems like even practitioners deem the activity as "higher risk".

@MastLind Is this easy move to learn? https://www.youtube.com/watch?v=gTA12kyB4IE

From what I've now read about "dune gooning" and assuming it's the same flyers in the two movies; I guess this is so hard to learn that those who really master it make it look simple.

It would seem that there’s a smaller margin for errors, and I could see it being quite tricky, especially if it’s gusty at all...

As should be obvious this was shot in Sydney, Australia. This type of flying is often referred to as Dune Gooning and it is great fun and not particularly dangerous (pilots rarely carry parachutes when engaged in this type of flying as they are too low for the chutes to do them any good if the glider breaks apart).
Getting close to the ground is, of course, the point. Skimming along an inch or two of it. Even a slight rise in the shore line allows for a upward wind component.

This does not provide an answer to the question. Once you have sufficient reputation you will be able to comment on any post; instead, provide answers that don't require clarification from the asker. - From Review

@ManuH One of the questions asked was "is it dangerous" and this definitely answers that. Maybe try and be a bit less enthusiastic with the "not an answer"-button?

@Sanchises I'm not the only one to have hit the "not an answer" button, only the first one. The review process exist so that such action are reviewed by at least 5 people. Moreover, the last revision when I was reviewing did not mention anything about danger.

@ManuH Are you sure? There's no edit history on this answer and your review comment was posted three hours after this answer was posted so well outside the quick edit grace period

@ManuH Regarding "reputation", you may not be aware that Davis Straub is a world champion hang glider pilot. Imagine that someone had asked a question about motorsport, Alain Prost happened to chip in, and someone downvoted him for not having enough reputation on the site? You've just done that.

@Graham This is a automated message, speaking of reputation on stackexchange. Argument from authrority is not valid as anyone can spoof an identity on the internet.

@Sanchises See revision of the question

It's possible, yes. I often see paragliders hovering in the air, thanks to the so-called 'ridge lift'. And yes, in that case, the wind, forced by the terrain, comes from below, at an angle...

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What are the locations on the Boeing 737 wing where the airfoil changes?
I would like to know where along the span of the 737 (let's say the NG) wing the two midsection airfoils are used. I am planning on running a CFD analysis of the 737 wing, and I need accurate data.

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What flights were significantly affected by insects hitting the fuselage?
Related
How do insects decrease aircraft performance?
This questions made me think was these effects actually noticeable in an any flights- maybe the pilots had to big changes to their flight management or did they reach emergency conditions?

Searching the NTSB database for "insect" returns 68 reports, 13 fatalities.

I was in a car that drove through a locust swarm (in Nairobi, in about 1986). Thousands of them smashed against the windscreen. It was if it had been smeared with a huge quantity of very viscous syrup. Visibility dropped to almost nothing, very quickly. It would have been a very dramatic thing to fly though, if you needed to see anything.

Are you limiting this to a certain category of flights? If not, then not being able to take off until the bugs were cleaned off should count...

@RonBeyer I got 148 returns.

Also search the ASRS database for insect or similar. Makes for some interesting reading, including several instances of insects affecting flight decision making or safety.

@RonBeyer how do you search the NTSB database?

@Pugz NTSB Accident Database enter as little or as much information in the form as you want, then submit the query at the bottom.

Went through a whole bunch of the "insect" hits on the NTSB site. So far they're all about insect debris clogging things - pitot tubes, fuel lines, vent lines, etc. I have yet to find anything about problems from insects striking the plane in flight.

Wasp induced pitot blockage was the primary factor in the crash of Birgenair Flight 301

Insects don't fly at 30,000 feet.

Add +1 to the "wasp in the pitot tube" comment. Flying in Louisiana, we were fairly cognizant of the possibility of a dirt dauber building a nest in a pitot tube or other open port. I've never managed to hit one with the pitot tube, but I have accumulated quite a few bugs during nighttime pattern flying and touch-and-goes. Even with a pretty gunked up aircraft, it never even caused a flutter. I wouldn't be concerned about bugs. Beyond the open ports concerns, anyway.

Paragliders and ultralights doen't fly at 30,000 feet

It is conceivable that an accumulation of bugs on the leading edge of an airfoil would affect its performance.  In fact, the in flight accumulation of bugs occurs at such a low rate, and only at low altitude, that it's not a problem.
Above a few thousand feet AGL, bug density is negligible.  A car, driving at night, or a small plane flying for hours at low altitude may accumulate a mess of bugs.  Even so, such change in airfoil shape and affect on drag is miniscule on powered aircraft that normally spend moments, not hours, at such low altitude, above which there is no bug density.
The common hazard of bugs hitting the fuselage is restriction to visibility through the windscreen.  Bugs on the windscreen can make it difficult to see other aircraft and creates a myopic focus point for the eyes.
Another conceivable problem is blockage of inlets and ports, through most certificated aircraft are designed with alternate systems, e.g. bypass and redundancies to deal with such emergency.  Flight into icing conditions, heavy rain and ash are of concerns here.

The link claims that insects affects the air flow due their very small size wings being effective at micro-level without necessarily accumulating. Is that claim garbage?

Bugs accumulate on the leading edge of airfoil surfaces and are of no practical concern.  Disturbance on the wing surface has potential to affect laminar flow, e.g. frost.

Is that effect significant- got to avoid a locust swarm might destabilise the plane or trivial the frost pattern changes and it makes gives no control issues?

I'm reminded of a question I asked my motorcycle instructor, "When should I swerve to avoid an animal?"  He answered, "If you can eat it in one sitting, hit it."  I still don't know what to do.

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Do the Mt. Everest rescue helicopters have modified engines to operate at high altitudes?
There are so many climbers on Mt. Everest today that there is now a helicopter rescue service there. They operate rescue up to 21,000 + ft. Because this is essentially stationary service it seems that engine modifications like a larger compressor section would be warranted.
In stories on the internet regarding these helicopters, none mention engines.
My employer had two Rolls-Royce RB211s operating at about 7,000 ft as stationary power and the power rating was cut in half for the altitude, so operation at 21,000 ft is impressive.

No, the helicopters are standard production versions. The Eurocopter AS350 is a common model used for these operations.
In 2005, Didier Delsalle landed a Eurocopter AS350 B3 on the summit of Mt. Everest at 29,029 feet (8848m) (twice). The only changes he made to the standard version were removing a few things like extra seats to reduce the weight, and of course flying solo. It also took careful planning and the use of updrafts to get there.
The B3 version of the AS350 (now called the H125) is marketed as high performance, designed for operation in hot and high conditions. It also holds the record for the highest long-line rescue mission at 25,590 feet (7800m) (in the Himalayas).
Although the official ceiling for hovering out of ground effect is 11150 feet (3400m), the flight ceiling is 23,000 feet (7000m), and rescues at 20,000 feet (6000m) are typically limited to 2 passengers rather than the full capacity of 6.
The Safran Arriel 2D engine used in the B3 version is rated up to 952 Shp on takeoff and operation up to 29,500 feet (9000m).

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Do the four engines of a BAe-146 make it efficient?
The BAe-146 is a small aircraft that can seat approximately 80 passengers, but it has four engines. Does having four engines instead of two affect the way it performs or is it more efficient or less efficient than if it were to have only two engines? I understand that it helps lessen noise during takeoff in populated areas, but is it more/less efficient in terms of both aerodynamics and fuel?

Voting to leave open, main question is about performance and efficiency.

The other question's answers discuss noise, climb-rate, short-field performance, redundancy and other topics but neither fuel economy nor aerodynamics. Question-closing mania strikes again.

Do the four engines make the BAe efficient? No! Does it affect performance? Yes it does. The BAe 146 occupied a niche for Hot & High airfields, in that it can continue the takeoff when an engine fails: one failed engine out of four leaves 75% of full thrust. So take-off performance with a failed engine is better on a 4-engine aeroplane.
The engines themselves were quite clever designs: the Lycoming ALF 502 with a relatively high bypass ratio and a geared fan. The bypass ratio of 5.7:1 is higher than the 5.0:1 of the Trent 700 used for the A330. By comparison, the Fokker 100 used the Rolls Royce Tay, with a bypass ratio of 3.0:1 and the proven core of the RR Spey.
 Photo from the wiki.
But was the BAe 146 efficient because of the four engines? For an aircraft of a given weight and a given thrust requirement, the design considerations for number of engines would be:

Four engines are each smaller than two engines. Turbine engines do not scale down well. All else being equal, purchase price starts to be the limiting factor when reducing the size: half thrust engines are not half the price.
4-engined planes have worse fuel economy than an identical sized twin engine, due to decreased efficiency of a smaller turbine, and increased drag from more engine pods and a larger vertical tail area.
In cruise, a typical single aisle airliner uses a third of the take-off and climb thrust. So once in cruise, one out of two engines can provide all the thrust required to continue the flight, and have less yawing moment than a failed outboard engine of four. Picture and quote from this document:

Specifically, current subsonic airliners require about three to five times more thrust to take off than they do to cruise, and the power produced by a high bypass ratio turbofan engine at constant throttle setting varies in much the same way.

Twin engine turbofans actually have a third engine on board: the APU. It can power the systems with full redundancy after an engine fail, and is normally shut off in flight, like the no.1 engine of a P-3 when loitering. Note that the propeller blades of the P-3 can be made to feather, providing much less asymmetrical drag than a windmilling turbofan.
So the advantages of four engines are mainly the consequences of an engine fail in the TO/climb phase: a four engined aircraft could opt to continue the flight, while the twin engine would have no more redundancy and a much harder time climbing to altitude. But all else being equal, four engines have twice the chance of failure than two engines...
The BAe 146 had a relatively wide and stubby fuselage, designed for 6-abreast seating but often configured for 5-abreast due to awkward headroom for the window passengers. This added to the drag penalty of the type over that of a comparable size aircraft like the Fokker 100, which is still a sought after aircraft for feeder airlines in Australia, the limiting and most expensive cost being engine overhaul. The BAe 146 has twice this cost...

Not to forget the maintenance cost - an engine inspection costs almost the same, regardless of size. Having four to inspect takes more time and costs more than inspecting only two.

Good point, have added.

Surely with more engines, you can have a smaller vertical tail, because losing an engine will have less overall effect on yaw.

@DanieleProcida You have to dimension the tail on losing the outboard engine.

@TomMcW The size of the engines is used as a justification for the number of engines, for propulsion in flight and also for powering systems etc. There is another engine on board that can power the systems: the APU.

@TomMcW Have rewritten the answer and deleted the comment to Penguin.


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Who pays airline pilots?
Do pilots make money from the airline they fly for (Delta, JetBlue, Southwest, Skywest), the company that makes the aircraft they fly (Boeing, Airbus, Lockheed Martin, Embraer, Bombardier), or just by the hours they fly?

Bus drivers don't get paid by the company manufacturing the buses :)

@J.Hougaard true.

Are you asking who pays the pilots, how their pay is determined, or both?

@fooot basically who pays the pilots.

Airline pilots are employed by the airline and are thereby paid by the airline.

Basically, yes, but it's not always apparent who "the airline" is. Who sells the tickets, owns the airplane, and pays the pilots could be three or more different companies.

Professional airline pilots are paid by the airline for their flight hours, similarly to other transportation workers.  Time spent in preparation for flight or even preflighting the plane itself is not usually compensated.  This pay varies by the company and the position (first officer, captain, etc.), and sometimes there are signing bonuses on top.  Pilots are given a monthly minimum of hours pay that they'll receive, even if they fly less than that. This is often as little as 65 hours guaranteed pay.
Regional jets, e.g. Republic, are typically contracted by another company like American Airlines. The regional airline, not the major airline, would pay the pilots for their flight hours.  This is even true if the regional jet has the livery of the major airline or the tickets are sold by the major airline.
Private/charter pilots get paid by the company (or person) managing the plane.  This can be either a per-day wage, a yearly salary, or a mix of both.  (Source)
Flight instructors are paid an hourly wage by their flight school.  This hourly wage does not include things like weather delays, your own education, and planning.   Flight instructors can also work independently and then they'd be paid directly by the student.
Except where noted, the source for this information is The Truth About The Profession.


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Position Calls Relative to airport
Hey I’m a new student pilot to just trying to learn some new things here. I’ve been having trouble in recognizing and communicating my position with cardinal points relative to the airport in uncontrolled airspaces as well as approach control. For example I seem to be disoriented when describing if I’m North, East, South, or West of whatever airport I’m flying into or from: I’m assuming it’s just the way I read the compass or heading indicator. But please let me know!

I don't think you should be asking here for such vital information and fundamental foundation building. Ask your instructor, ask someone qualified. This site allows for any random joe to mislead you and put you at risk if their actions go unnoticed.

If your nose is pointed exactly at the airport, then your bearing from the airport is 180 degrees off of your heading- just read the opposite side of your heading indicator or add/subtract 180 from the number on your compass and it will tell you which direction from the airport you are.
If you are not directly pointed at the airport, you should first figure out what heading would take you exactly to the airport. If the airport is at your 2 o'clock (that is, 60 degrees to the right) and you are flying a heading of 030, then a heading of 090 would point you directly at the airport. So you are at a bearing of 270 from the airport, due west.
Keep in mind that this is something you can and should figure out before you ever get in the plane! When you plan your route to another airport you should be able to figure out what your position relative to the airport will be as you get close just from your plan.
You should endeavor to have a mental picture of your situation and your position at all times. To some degree this will come with practice. But if you are having difficulty tell your flight instructor so you can work on it together.


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Does Green Dot Speed Calculation include banking Angle?
I'm wondering, whether the Green Dot- speed calculation in the Airbus PFD does account for the actual bank angle of the aircraft.
Green Dot Speed is the speed for the best lift to drag ratio as it is stated in Safety first SPECIAL  EDITION: Control Your Speed Series.
In that edition, it is stated, that the Green Dot-speed is a function of aircraft weight, which is quite obvious.
I wonder however, whether the current roll angle is also accounted for as the hodographs like given in Avrenli, Kivanc, und Barry Dempsey. „Is “Green Dot” Always the Optimum Engines-Out Glide Speed on the Airbus A320 Aircraft?“ The Journal of Aviation/Aerospace Education and Research, 2015. suggests:

Any ideas how to get those numbers for Airbus aircraft?

You can find a table from the QRH in this answer to What is an airliner's “green dot speed”, and how does it vary with weight, and why?. The table is only as a function of weight though. It does not say if the actual code in the aircraft takes bank angle into account.

@Bianfable: I have seen the referenced QRH, but honestly I need some advice how to read it: For which aircraft are the tables valid? E. g. on p. 82 there is a tableand a very similar one on p. 84. (There are plenty more.) For which aircraft are these tables valid? Maybe you can tell me the trick on reading the QRH. (I'm not a pilot, but a programmer:-( )

That is probably for the different A320 variants (A318/319/320/321). Details will depend on the exact QRH you have (they are usually somewhat different for each operator). Does the page maybe list the variant or engine somehwere?

The Green Dot speed is very stable on the PFD, unlike for example VLS which is a function of G (bank included), mach number and speed brake position.
According to Flight Crew Ops Manual(s) green dot speed varies only as a function of aircraft weight and altitude. For A320 a very crude formula is given: below 20000ft 2 x weight in tons + 85kts, above 20000ft add 1kt/1000ft.
Based on experience I would say FCOM is accurate on this matter. It wouldn't be the first time the whole truth is not revealed to the pilots, though :)

The PFD is very stable on PFD What did you mean to say there?

I meant that it does not change as the aircraft is maneuvered, ie it fixed on short term. (it takes a while for weight and/or altitude to change)

@busdriver The first sentence has "PDF" for both nouns.  That is what we don't understand.  Did you mean for the first noun to be something other than "PDF?"

In theory, GD is 1.3 times the stall speed. The formula is:
$$ V_\text{GD} = \sqrt{\frac{2 m g}{C_L r S}} \times 1.3 $$
Where:
$m$ is mass, $r$ is density, $g$ is gravity, $C_L$ is max coefficient of lift and $S$ surface of wing
In level flight $g = 9.81 m/s^2$, but when banking $g$ should increase, therefore the GD should (in theory) go higher in the speed tape. At least that is what happens in the E-Jet. In this aircraft GD = max climb angle and best L/D. Also GD in the E-Jet accounts for ice accreation (mass increment in the formula). I hope this helps.

I just watched a video of Embraer as they obviously are somewhat more transparent on their calculations than airbus is, but there they explicitly state, that bank angle is not taken into account for the Green Dotcalculation. https://www.youtube.com/watch?v=EphvGccIwgA ~ Minute 0:50. So I wonder whether your described behavior is "what it should be" or "what it actually is".

@opt12, yes you are right. That's in one of the many videos we saw in the company long ago. It's really not updated. With the new software loads (I think we are now in software load 27. something) the calculation has changed a little. For example, the GD now accounts for ice accreation and, if I'm not mistaken also g load. I will check it out next time I'm in the sim :)


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Would knowing the exact weight that has been loaded on a commercial plane and it's distribution be useful?
There's been a bit in the media about weighing passengers in order to get more accurate take off weights for commercial planes.  I'm guessing the delta between a heavy set of passengers and a light set might be 3000 kg.  If there was a cheap way of determining the weight that has been loaded onto a commercial jet would it be valuable out not, given this level of weight difference? I am after an actual cost. What is the cost of an airliner carrying the
additional fuel required by current rules if it assumed the overall mass estimate is 3000kg too high throughout its life? It won't be using that fuel but it will have to carry it.

"current estimations are enough" ???  Investigations need facts, or, better yet prevent the accident before it happens.  This question is not only about weight, but also distribution.  It does not seem to be a duplicate.

A very important consideration would be adequate fuel reserve safety margin for headwinds, go arounds, unanticipated delays, etc. as compared with total weight of aircraft.  A study of frequency of fuel starvation incidents with current margins would also be in order.

Ymb1 - I have edited the question to make it clearer. Thanks

YES.
The obvious solution is to roll the plane over a scale.  Useful CG data can also be obtained by weighing the front wheel and comparing its torque to known values.  A light front wheel means an aft CG.  This could be corrected by moving fuel BEFORE the plane is airborne, as well as while in flight.
Of course, moving passengers and cargo are an option too.
For example the plane weighs in at 2200 lbs total on the ramp ready to go.  The nose wheel weight is 200 lbs.  The distance between the main and nose wheels is 11 feet.  The aft CG limit is 0.75 feet forward of the mains.
CG is calculated as follows:  200 lbs/2200 lbs = 1/11.
          CG is 1/11 * 11 feet = 1.00 feet forward of mains.
          CG is with in limits.
And how accurate weighing the nose wheel can be (due to long torque arm):
Nose wheel weight 100 lbs.  Total weight 2200 lbs.
        100 lbs/2200 lbs = 1/22.
         CG is 1/22 * 11 feet = 0.500 feet forward of mains.
         CG is NOT in limits.

Technically nothing incorrect here, but the question had to do with larger commercial jets.  2200 lbs is around the weight of a Cessna 172, and yes, small planes are very sensitive to CG as well as overloading.  I agree with Ymb1's answer on the duplicate question linked above.

@Michael Hall I drive a 40 ton tractor trailer.  We go over scales. As a matter of fact, this approach is "scalable" through a wide range of types.  The benefit is being able to do gross weight and CG with some simple algebra.

And I promise I will not attempt to answer authoritatively on any questions pertaining to 40 ton tractor trailers.

I agree with you, but if it is truly a good idea then why aren't airlines doing it?  Ultimately it is a business decision based on cost vs benefit.  Driving over a set of scales costs precious time and fuel to verify with just a bit more accuracy what you already know.  Also, if it was important enough to safety then it would likely be mandated by the FAA.  It isn't like nobody has thought of it before...

@Michael Hall OK, as a comment (answers should be authoritative), 3 reasons.  Aircraft are being designed to tighter aerodynamic specs, yet a huge amount of variables such as runway length, weather, traffic patterns, payload, and CG remain.  As planes are tweaked to perfection, their tolerance to these variables may be less.  Airports designed in the last century also need to keep pace, and gov't regulations for Passenger Aircraft could be such that over competition by Majors for the most "efficient" planes will not lead to danger.  Weighing and fuel transfer preflight could make it safer.

Then my advice is to invest in a system of scales and make your case to industry.  Again, I am confident that this has been considered and decided against by professionals with a lot more specific knowledge on the subject than either of us.

For cargo aircraf,t going over a scales and thus having the weight on the individual landing gears is not going to allow you to check the linear limits, the fwd and aft cumulative limits (sometimes referred to as the monocoque limits), and the individual position limits. It also would not allow you to make maximal use of asymmetric limits position limits for wide body aircraft.

@Terry. Look at a triple beam balance. Just weighing the nose wheel would tell you a lot.  Most critical is CG position.  For cargo aircraft it has also been proposed to slide the cargo bay floor over a simple roller set at optimal CG until it tips.  Getting CG right can help if the designer wants a smaller tail.  But weighing the individual pallets as they go on and doing a proper load sheet certainly will work too!  Weighing the nose wheel only double checks it before flying.

And BTW, if we bothered to build a scale house (@Radu pointed out wind is a factor), we might get a little de icing done too.


text:
Why doesn't Su-57 blend the engine nacelles with the fuselage, like other 5th generation fighters?
All the current and upcoming 5th generation fighter aircraft have increasingly similar physical layouts regardless of their origin. This seems to be due to similar design choices around stealth and aircraft performance. Refer to the image. All of these aircraft have a "box" fuselage, blending the engines, fuselage and wing roots, engine intakes located on the sides of the main fuselage, high-mounted trapezoid wings, slanted out twin vertical stabilizers, and the like.
The Su-57 still uses the typical Su-27-like engine layout, which has many more edges and corners, potentially making it more challenging for a stealthy design.
Why does Su-57's design prefers this layout, while all other 5th generation fighters go for a blended "box" cross section of the fuselage?
Edit: Based on the answers, it seems that Su-57 suffers from a higher RCS due to this and other design choices. The theme was to build up on the Su-27 platform instead of developing a totally new platform solely for stealth like F-22/F-35 or J-21. One answer even goes as far to not consider Su-57 as a 5th generation fighter, which seems logical.

Edges and corners don't in themselves reduce stealthiness.  If anything, stealthy airplanes look more like origami than non-stealthy ones.

This answer should help

@sophit the accepted answer on that question seems to put roughly the same point I am trying to make. Having less number of edges and bends helps in stealth. However, Su57's distinct nacelles increases the reflectivity. The sharp edges near the intakes may even be very good corner reflectors. "With better computational methods, also curved surfaces could be simulated and it became possible to reduce edge count. But ideally the surface should be horizontal near all edges, which explains why fuselages and wings look like they had ironing edges."

Russian designs are most stealthy in the forward aspect angles. Side and especially rear angles receive much less attention. The thinking is to sneak up on the enemy, but once the bombs fall or missiles fly, stealth is moot anyway.

@PeterKämpf The exposed fan face on the Su57 increases RCS from certain front angles by 10x, hardly a way to sneak up on anyone.

@Pilothead It's not exposed, there's a shallow S-duct with radar blocker grating inside. Not as good as deep S-ducts, but consistent with limited stealth.

@Therac The radar blocker is similar to that used in the F18, which isn't that good.

There are different definitions of what to call "stealth". Some aircraft like the B-2 are total all-aspect stealth, others like the F-15SE proposal only make small RCS reductions.
The Su-57 is in between. It's a stealth design, but it focuses on forward aspect stealth. This is similar to the F-35, which also forgoes rear hemisphere stealth. The Su-57 applies major RCS reductions, like eliminating vertical surfaces and shielding the engines, but still builds upon the Su-27 Flanker platform, so they couldn't completely rearrange the planform.
The reasons are historical. The Soviet Union's program to develop a 5th-gen fighter started in 1979, 2 years before the US ATF program (the F-22). Both programs wanted a low-observable, supercruising, supermaneuverable, fused-sensors fighter. Here is what the US considered:

The USSR also had all its design bureaus compete, but I can't find open sources for their designs. In 1987, Mikoyan won the competition with their Project 1.44. Sukhoi, Yakovlev, Ilushin, Antonov, Tupolev lost. In 1991, the USSR collapsed, and the program was defunded. Russia was considering joining NATO, or disarming completely.
Meanwhile, Sukhoi's Su-27 was raking in the cash. It briefly overtook even the F-16 as the most-exported fighter, though not for long. Heavy fighters usually suffer from poor sales, but the Flanker's fly-by-wire, supermaneuverability, payload, reliability made it popular for export.
Sukhoi knew the future is stealth, and invested into developing their own design further. In 2000, when the new government revived the 5th gen program, Sukhoi had a prototype ready to go. The program took just 10 years and $1 billion from that to first flight in 2010.
The only way Sukhoi could accomplish this was by reusing the Su-27 as much as possible. Early Su-57 prototypes were just modified Su-27s, testing one thing at a time. Later, they changed the airframe, but they had to keep the overall layout.
Some prototypes tested flat combiner nozzles, suited for deep stealth. Some had complete S-ducts and blended nacelles:

None were picked for the production version. S-ducts are heavy. IR-lined flat nozzles are even heavier. This results in a notable drop in performance. The F-22's useful load is only 0.48 of MTOW, down from 0.58 for the F-15. That's a massive drop, and part of why the F-35 lost the stealth nozzle. Sukhoi didn't put as much trust in stealth as Lockheed did, and wanted a plane that would work well even if stealth failed.
The practical fact is that the Su-57 is a transitional design, rather than a new all-stealth concept. That's not necessarily a bad thing - stealth is as of yet untested in peer-level combat, and it can be countered. To that end, the Su-57 has the most comprehensive counter-stealth sensor suite to date, with two IR systems and a dual-band radar, which might offset its higher RCS.
From the top, the Su-57's design (left) is consistent with stealth, if following the concept of low-level penetration. But Sukhoi's all-stealth design (right) is different:

The Su-57 makes compromises. It can take different engine models, be quickly modified for A2A or A2G, uses intake radar blockers rather than deep S-ducts. Stealth-wise, while the B-2 and F-22 are estimated at roughly -25 dBsm median RCS (-40 for head-on aspect), the F-35 at -20, the Su-57 only claims around -10 dBsm median. That's a lot better than +10 to +15 of most 4th gen fighters, but it's not state-of-the-art stealth.
The Su-75 (on the right), slated to fly in 2024, doesn't compromise. It's designed from scratch, has no predecessor, follows every rule of stealth design there is. It was started in 2000s Russia, enriched by high oil, global trade, and Sukhoi's own export success. Yet, it doesn't yet have a single customer signed up.

The reasons are trust, reliability, familiarity. It will take a long time to get acquainted with the Su-75 and iron out all the kinks. Even longer to learn how to keep it stealthy - a challenge in itself, which took impractical restrictions for the B-2 and time for the F-22. India and China realize it will take a long time to learn, and China's more eager to invest that time in their homegrown platform.
On the domestic side, there's the cat-and-mouse game between stealth and counter-stealth. Playing is definitely better than not. But Russia has previously claimed the S-300 to have strong counter-stealth. If true, it means the Su-57 and even Su-75 won't be able to rely on stealth in former Soviet countries, which are stock-full of S-300's, originally meant to guard against US/NATO. This might be why the compromise option was picked for the PAK-FA over full-stealth prototypes.
Also, it could be that the shape works better than it seems to. But Sukhoi's own patent only claims -10 dBsm. Here is an independent evaluation of the Su-57's RCS, compared to F-35 and J-20. The conclusion of this modeling is that the Su-57 has about 10 dB worse RCS than the F-35, and its engine placement plays a role in that. It is, however, a major improvement over 4th gen fighters.

Summary: Sukhoi hedged their bets with the Su-57, building a plane that can sell as either a stealth fighter or as a better Flanker. It doesn't look as stealth-oriented as the F-35, J-20, or Su-75, because it isn't. But from key aspects, its RCS reduction is significant, about 100-fold.

Thanks. Can you please give the source of the RCS numbers? Also the source of "Sukhoi itself says it's a compromise".

So you mean to say they didn't want it to be stealthy as bad as to try out a new new layout, they just went with the old flanker design?

@pakieng Stealth on a tight design budget isn't easy. By starting with the Flanker, they got a plane that works right away, stealth or not. Plus, they were able to reuse Su-37 parts and rigging, until new models were developed. Early Su-57 prototypes were just modified Su-27s, testing one thing at a time. The old source went down, I'll look for one.

@pakieng Linked the source, at the end.

Thanks for the source Therac

The F22 uses blended airframe that was designed for stealth very onset.
Russia knew stealth was a terrible design alone so sacrificed capacity in exchange for improvements of engines. More likely nacelles are easier to open to access engine mechanics and part replacement befitting Russian simplicity philosophy. And being slightly larger, the engines are straight and ducts largely straight as possible.

Interesting!! Looking from the front ±10 degrees, you could look straight on to the engine blades. Terrible terrible for stealth.

@pakieng There's an angled duct and a radar blocker in front of them.

It has lower observability, but isn't really used like a stealth aircraft.
The whole point of stealth aircraft is that they penetrate contested airspace: See the Israeli operations over Iran or the F-117 operations in the Balkans, or the B-2 strikes at the beginning of the second Iraq war. If the Russians thought the Su-57 was a stealth aircraft and were right, we'd be seeing it used to strike targets in Ukraine with impunity, like Israel's operations against Iran or the US's operations in the first stages of the second Iraq invasion. If the Russians thought the Su-57 was a stealth aircraft and were wrong, we'd be seeing losses over Ukraine, like the F117 over the Balkans.
Instead, the Russian deployment choices only make sense if they are perfectly aware that it's not a stealth aircraft, and are taking this awareness into account.

Interesting comment, but I'm not sure that this answers the stated question: why aren't the engine nacelles blended?

Stealth doesn't give you impunity, only some tools to avoid detection. The Su-57 is being used cautiously, but it only has frontal-hemisphere stealth for air-to-air, not all-aspect as required for deep interdiction.

The F-117 loss over Yugoslavia was due to operational errors rather than the aircraft itself. Everyone with an ounce of sense understood that stealth isn't a guarantee you won't be seen, but the pilots got overconfident after practically overflying multiple SAM sites and not being painted so they got complacent and allowed themselves to be lured into a trap. An F-22 or F-35, or even a B-2, would end the same way if used the same (their stealth signatures also completely blossom into lighting up every radar screen in a wide area when opening their weapons bays).

The simple answer is the Su57 is not a fifth generation fighter. Stealth is arguably the most important characteristic of fifth gen fighters, and the Su57 is not stealthy. It has the RCS of an F18, or about 1000 times more detectable than an F35. Global security lists the F18 at 1sqm, within the range claimed for the Su57, and the F35 at .005 or -30dbm. From the front at certain angles it even has a 10x higher RCS spike due to exposed engine face in the air intakes. Marketing hype does not change reality.
This doesn't mean it is a poor aircraft that isn't useful. It was just a low budget attempt at stealth that failed. Militaries are still buying upgraded fourth gen aircraft, but anyone trying to conduct missions that require stealth using an Su57 is in for a rude awakening.

The first source is very poor. First, it distorts the facts: Sukhoi's patent refers to average RCS, which can't be compared to frontal RCS. Second, from no angle is the F-18 a 0.1 m2 RCS target. Third, it contains flights of fancy like -50 dB RCS for the F-35. Fourth, it cites Quora answers as its sources.

@Therac So instead of complaining about my source, offer your own comparison of a Su57 RCS to an F18 and F35. This is not the only place where this is stated. Global security lists the F18 at 1sqm, within the range claimed for the Su57, and the F35 at .005 or -30dbm.

Here is the F/A-18's RCS modeled. The Hornet has a rounded all-metal fuselage, its sides light up to +15 dBsm and more.


text:
finding the approximate course change
I'm studying for my nav test (EASA ppl) I will be thankful for a good explanation solving this
:
The distance from 'A' to 'B' measures 120 NM. At a distance of 55 NM from 'A' the pilot realizes a
deviation of 7 NM to the right. What approximate course change must be made to reach 'B' directly?
a) 15° left
b) 8° left
c) 6° left
d) 14° left

Have you come across the "1 in 60" rule in your studies? That should help you solve this one

This requires using the 1 in 60 rule.
This rule states that if you have flown 60 miles and become 1 mile off track, your track error is 1 degree.
The rule is based on the small-angle approximation (which states that, for small angles, sin θ ≈ θ, where θ is in radians), along with the fact that one radian (which is about 57.3°) is close to 60°. In reality, a 1 mile in 60 error is 0.96°, and the rule becomes increasingly inaccurate for larger errors. It states that for each degree off (or displacement) over a distance of 60 nautical miles (NM) will result in 1 NM off course. Plug this in for this question too.

thanks guys ! I now understand the principle of it


text:
How do you contact ATC if you want to do RC flying in the US?
This question talks about flying RC models in US and latest "guidelines" to follow.  One of them is contacting ATC tower if you are within 5 miles of the airport
The seems pretty simple but I'll ask anyway:
How would one go about contacting tower to let them know you are going to be flying RC near airport?
Any specific verbiage that needs to be used?
Can the request be denied?  (Aside from TFRs obviously.)
Does anybody have any actual experience doing this?

Regulations have changed greatly since this question was asked and since the last answer was posted. For example, there is not longer a requirement to contact the airport or control tower when flying within 5 miles of an airport, but there is a requirement to use the LAANC system to get "prior authorization" before flying in most controlled airspace. There is also a blanket maximum 400' AGL altitude limit.

Any specific verbiage that needs to be used?

Just tell them you will be flying an RC aircraft with 5 miles of their airport. You should have on hand the location you have in mind. Preferably, know the distance and bearing from the airport, like "3 miles to the southwest". Even better would be a more exact bearing and distance from the airport or a nearby VOR. Also know what max altitude you expect to be at. See the related question for a more thorough description (about NOTAM info, but the same general idea applies): How do I submit a NOTAM?
And if you are wondering how to get in touch with the tower: How can I find telephone numbers for FAA ATC facilities?

Can the request be denied?

In general I would say yes. One of the main rules of RC flying is to not endanger manned aircraft. If ATC thinks your selected location will break that rule, then you shouldn't fly there. That would be part of why they ask you to contact ATC in the first place.
However, the wording is "provides the airport operator and the airport air traffic control tower...with prior notice". It says nothing about receiving approval, clearance, or permission from them. So I'm not sure if you are breaking any regulations if they say no and you fly anyway, assuming you don't break any of the other rules.

Does anybody have any actual experience doing this?

Not me at the moment. I will try to update if that changes.


text:
Why did an iPad software failure cause a return to the gate?
I was amazed to read this which implies that a software failure on iPads caused departing flights to return to the gate.
I used to be an avionics engineer up until a point before the current generation of consumer electronics and now develop "field" apps for phones and tablets.  I know two things to be true.

Consumer phones and tablets of all persuasions used in field applications have nowhere near the reliability of certified avionics.  I own an iPad, amongst many
other devices, and it does not get close to six nines reliability (as do
none of my other consumer devices).  In particular, since the latest iPad updates over the last year or two, it's been quite buggy.

The hardware is not designed and built to the standards required for avionics and, without appropriate hardware, the
software cannot be to aviation standards either.

I know that these devices have been certified for use and I know why and what they are used for but I thought they were only convenience devices.  It seems that they have become part of the safety chain.
Are they MEL items?  Does anyone know what's really going on here?

Everything that you say in your question is correct, and to address your reliability concerns, as part of certification the FAA requires certain hardware testing (EMI testing, decompression testing, etc.) of any EFB, including the iPad.  They are not however, certified to the same standards as avionics, because a failure of an iPad does not create the same safety issue as other certified parts.
Beyond the testing for certification, there is required testing by the certificate holder before any update may be applied at the end user level, and part of the approval process requires written contingency procedures that are approved by the FAA for use in the event of a failure of the iPad(s).
Typically, at least two are required in the cockpit for dispatch unless the paper equivalents of all electronic documents that they are approved for are available (charts, operations manuals, AFM's, flight logs, etc.).  If they failed prior to takeoff, this is likely why they returned to the gate.  There are contingency procedures for an in-flight failure of a single iPad, and for failure of all iPads on board.  These are written by each applicant but typically require "anticipated information" to be manually recorded from the working iPad, or using another source (FMS, ATC, data link, internet, satellite phone, etc.) to get the information if they both fail.
The FAA has a ton of guidance/requirements available for those wanting to use COTS hardware for EFB's.  Also keep in mind, that using an iPad as an EFB is only a few years old, but EFB's have been used much longer than that.  Typically in the past, a Windows based tablet computer was used (talk about reliability issues), and the procedures so far have worked quite well.  I don't know of a single accident caused by an EFB failure.
Whether or not the iPad is on the MEL depends on the class of EFB that it is certified as.  If it is certified as "carry on" equipment, then it is not required (other than by company policy) so will not be on the MEL.  If it is "installed/mounted" in the aircraft, then it may be listed in the MEL.
For lots more information, see:
AC 120-76C - Guidelines for the Certification, Airworthiness, and Operational Use of Electronic Flight Bags

Thank you.  The best answer so far.  I am not concerned about reliability, not having charts and so on is not safety critical the fact that hundreds (thousands) of customers were inconvenienced, some no doubt financially, due to reliance on a piece of COTS is what amazes me.  Anyway, thanks again,

Prior to the US Airways - American merger, 2012 or so, American pilots would return planes to gates over maintenance issues where they had the discretion to do so, as a form of work stoppage in their ongoing collective bargaining negotiation (http://www.cbsnews.com/news/pilots-grounding-american-airlines-without-strike/).  Whether this indicates a return to labor trouble or not, it may be more precise to say the pilot elected to return the plane to the gate based on the iPad event.  Having experienced a 3 plane change at the time in Ft Worth, "customer inconvenience" is not the issue.

Does that mean that if a 0-day vulnerability is found in iOS, those aircraft will have to keep using the vulnerable version until they can get approval to upgrade to a version without the vulnerability? Does the contingency plan assume the pilots know there is a problem with the devices? If an attacker were to compromise the devices they could present the pilots with corrupted but realistically looking data.

@kasperd There are multiple, redundant sources of information involved, and no single point of failure should create a major hazard, including this.  The aircraft navigation databases, the charts, and ATC all need to be on the same page, and if there are conflicting sources of information, they will be resolved using the other sources.  To answer your first question though, any new version has to be evaluated (although not as extensively as the initial version) so most likely they would revert to procedures that assume iPad failure (use paper) until a good version is available.

iPads are used so the flight charts and manuals are electronic instead of the old phonebook that had to be printed and distributed every few months.
This means that without it the pilot has no up-to-date copy of the flight chart of the airport where he is trying to land which is a safety issue.

Shouldn't they have backup iPads and/or paper charts?

@SpongeBob the iPad is meant to prevent the waste of paper that updating the paper charts pose. And what can affect one iPad can also affect another (especially when it's not often used like a backup would be).

I do not understand why this answer has been downvoted: it answers the questions and does not contain mistakes. What's the problem?

@Federico  I'm not sure it does answer the question (however, should not be downvoted).  My question is effectively how has a piece of consumer electronics got into the safety chain.  I understand why they are used and what for but clearly, there is a weak link.

@RedGrittyBrick: No, but a bad weather diversion on a coast-to-coast flight already puts you in potential range of hundreds of airports.

@Simon how has a piece of consumer electronics got into the safety chain, well, you said that you know that these devices have been certified for use. that's how.

@Federico  To be fair, that's kind of saying "because that's just the way it is" which is not a great answer ;)

@Simon, actually my point was that the question, imho, is not well posed, if you want to know how the ipads got through certification you should reword it, or the answer to how they got into the loop is and remains they passed the certification process

@Federico My question is clear I think but please do edit if you think it could be improved - Are they MEL items? Does anyone know what's really going on here?

So if the iPad fails in flight, the pilots don't have the charts necessary to fly the plane? Does that mean the iPad has been certified under avionics standards?

@raptortech97 The short answer appears to be yes, or at least that the FAA is prepared to approve a process which uses an iPad in lieu of paper charts.

@NigelHarper Thanks. I'm literally shocked by this. I can only assume that the FAA will be reviewing the certification. I get that there isn't much of a safety risk since not having charts does not mean that a safe diversion and landing is not possible but does add a very weak link into an otherwise strong chain. In particular, I smiled when I read this - the app proved “extremely stable” during testing. In the “unlikely” event of a software crash, he says, it takes but a moment to get them running again. Clearly not.  It's a consumer device, on  second thoughts, why am I shocked?

@Simon note that the wired.com article is from 2011.

@raptortech97 I have added an answer with information regarding certification and contingency procedures for you which explain some of your questions here in the comments.

Many of the answers here are excellent in general.  Maybe I can help flesh out some of the details.
My understanding is that American had issues with the application from Jeppesen through which they distribute the charts used to navigate.  In this case I believe it was the airport diagram for DCA which conflicted with an earlier version on the iPad.  This caused the iPad to unexpectedly crash and the app had to be deleted and reinstalled to fix the issue (a common fix on the iPad).  This is a bunch of data to download all at once, especially by multiple pilots over a wi-fi connection.  I suspect that increased the delay.  (Source plus personal accounts)  This is the electronic equivalent of every affected pilot spilling coffee on their entire set of charts and manuals at the same time.  As others have said, if you don't have manuals and charts, you legally can not leave. That's the core reason why these planes had to return to the gate.
We use the same app (didn't have this issue that I know of) and the same hardware.  It has a lot of benefits for the pilots and the airlines.  However, in addition to the concerns that Lnazfiger raises, there are a bunch of others especially when the COTS product is the iPad which was never designed for enterprise use.  This creates a bunch of issues

Apps and data can be pushed out onto the device without solid
testing
iPad has no backward compatibility with OS changes and
the user can control OS updates. This often breaks the apps being
used if they aren't yet updated to the new OS.  The company can't
prevent this
The user has immense control over the ecosystem,
opening it up to risks, threats and user introduced instability
These systems weren't designed for this kind of data or these
applications and, until recently, Apple approached the
implementation of these devices on an enterprise basis as if they
are simultaneously selling thousands of consumer iPads.  My
understanding is that as sales drop off, they are starting to look
more at enterprise deployments but they wanted little to do with the
carriers originally.
The application ecosystem must be tested by
the carrier to ensure that there aren't conflicts between apps and
data.  Apple does a nice job of separating apps but that doesn't
mean issues don't or can't occur.
Some of the companies producing
these apps are not originally software firms (e.g. Jeppesen, WSI,
Boeing.)  Some do a great job but I know that my experiences with
the same Jeppesen app aren't stellar.  Nothing this way, but I've
had issues I consider sloppy.

Of course, these same risks present opportunities for enterprising software engineers and it's improving as the market improves but all of the risks and these are still present.  At the same time, the benefits still outweigh the risks.  I suspect that companies like Boeing and Jeppesen having their name in the press attached to issues like this will be pressed to fix any issues quickly.

Some of the issues that you bring up here can be solved by the use of MDM quite effectively, and indeed I have used it to control updates, etc. rather than allowing the pilots to update this when they feel like it.

Agreed theoretically.  It's probably the way that we have implemented it, but many of the issues still remain.

I know from painful experience that no MDM I am aware of can prevent the user from applying OS updates.  It's part of the providers strategy and they expose no mechanism to 3rd parties to prevent them.

Which is unfortunate in an enterprise setting.  I understand that there are ways around it (don't know much detail) but the cure can be worse than the disease).

Nobody knows exactly what happened, but as far as I can understand the limited information available, this is what happened:
These iPads are used to carry map data that the pilot needs, and which used to come in the form of forty pound of paper. That data needs to be updated from time to time, and it is supposed to be updated through WiFi automatically when the pilot is in the cockpit at an airport.
That automatic update didn't work for some reason. The pilot found that the map data wasn't up-to-date and therefore wouldn't take off. Instead he took the iPad and carried it into the airport, where the map update worked without problem and then he took off with an iPad with good map data, with a delay.
So it seems  that this was highly inconvenient, but not a security risk, assuming that the pilot used his checklist as he should. And the co-pilot had exactly the same problem with a second iPad, and fixed it the exact same way by carrying it to the airport. So it is unlikely to be a problem with that iPad.
PS. These iPads can be connected to power in the cockpit, so there is no fear that their battery power would run out.
PS. Both pilot and copilot have an iPad, and they really only need one, so you could count that as backup. I read (but I have no idea if it is true) that there is supposed to be a third set of the maps for the airplane as well, so one set for pilot, copilot, and airplane each.

Hi gnasher, welcome to Aviation.SE. Do you have citation for the automatic update via WiFi at airports statement above and the accounts of what happened?

I don't believe that this is the case.  All of the reports I've read (and of course, none of which can be relied upon) indicate that is was a software failure.  Anecdotal quotes from the pilots suggest that the "screens went blank".  Nor did I suggest any kind of security issue.  However, the need to return to the gate suggests that they are indeed MEL items, if they are not, then why return?  Since they failed for all pilots, the point about having 2, or 3 (or 20) onboard is moot since the software failed.

Such redundancy only helps as long as shared fate can be avoided. What would the consequences be if a security exploit allowed an attacker to feed identically but corrupted data to both devices through WiFi? The only hardware needed to perform such an attack would be a laptop, which many passengers carry on the plane. A passenger could unknowingly have picked up a piece of malware, if the passenger got the malware from a rogue access point located close to the gate, it might even be possible for an attacker to target specific flights.

@SentryRaven: It's a conjecture. But it is consistent with the fact that they had a problem while in the cockpit, which was fixed by taking the iPads into the airport.

@kasperd: It is probably quite easy to interfere with WiFi (stop it from working). Could happen by accident, if someone on board has a computer with broken WiFi that transmits all the time. But if the developers were not completely stupid, then the updated data would have to be encrypted, signed, or both, and making the device accept forged data should be impossible.

@gnasher729 They are using a software stack, which is not designed to the same standards as the rest of the plane. Whether it has vulnerabilities or not is really not part of the question. It is however very relevant to the question what the consequences are if such a vulnerability exists. Can security be jeopardized by an attacker who can place corrupted data on both devices with the data being constructed in such a way that it is not immediately obvious that it is corrupted?

That's close.  The data actually did update, but one of the new records conflicted with an old record, locking up the application.  For some reason,    as far as I can understand from some involved, it locked up the whole device forcing them to reload the application stack.


text:
Why do I not always hear ATC read back by pilots (on required instructions)?
I enjoy tracking air traffic at my local KORD.  I listen on LiveATC and use my private virtual radar setup to get "real-time" traffic info.
I understand which instructions need to be read back by the pilots per this question however on more than one occasion I don't hear read back on critical vector info on departure, despite the visual confirmation of instruction (pilot making proper vector and speed adjustments).
I tend to notice this with bigger birds (777,747,340), however smaller regional jets almost always promptly read back.
Questions:

Is there an alternative way of ATC instruction acknowledgement? (Other than read back?)
Is it possible that the reply is somehow on a different frequency?
Is this just a problem with LiveATC? (One theory is that A/C leaves receiver coverage area and that's why I don't hear reply, however on approach side much bigger distances are heard in my area)

Thank you

I did verify that indeed the aircraft that I don't hear read back from receives special departure frequency by tower.
Once tower clears aircraft for take off and hands them off to departure controller they use "departure" frequency to read back instructions.  It is typically different from standard departure frequency.
The only thing that is not clear is if there is any rhyme or reason for which aircraft gets this special departure frequency and which doesn't.
As pointed out in the question for some reason mostly it is "big" aircraft that gets this preferential treatment, but I am not 100% sure why.

I can't speak to KORD specifically, but it is possible that the read back is on a different frequency -- if you're not hearing any of that aircraft's calls. It's not typical for exchanges to be on different frequencies, but it's not unheard of for ATC to be on multiple frequencies (such as one you're listening to and another) while the aircraft is only on one.

In some situations ATC is transmitting on multiple frequencies and you will only hear replies from aircraft on your freq. You may also be having line of sight issues with antenna placement.

I heard on couple occasions departing aircraft receives departure frequency from tower, is that the frequency on which they read back on or is that just for take off clearances?

Also if the craft is a military and the airport supports it, ATC is talking on both the VHF and UHF channels and the military craft is replying on UHF. Military craft have the capability for talking on civilian frequencies, but are more comfortable sticking to their own.

The O'Hare Eight Departure is the only departure procedure out of ORD.  This is what it looks like:

Notice that there are three departure control frequencies:

Chicago Dep Con West: 125.4
Chicago Dep Con East/North: 125.0
Chicago Dep Con South: 126.635

Airplanes are usually given the frequency appropriate for the sector that their first departure fix is located in.  If they are very busy, each sector will have its own departure controller and they will be talking to different people.  If it isn't as busy, they pilots will still be talking on their individual frequencies, but one controller may be listening to and transmitting on two or all three frequencies at once.
For instance, if a particular aircraft is cleared over POLO VOR (due west of ORD) they will be given a departure frequency of 125.4 as part of their clearance.  Let's say that the traffic is slow and only one controller is being used to cover all three frequencies.  If you are listening to the East/North frequency on 125.0, you will hear the departure controller issue a clearance to the aircraft, but never hear the response because the aircraft is transmitting on 125.4.

So, the one specific instance when this happened last time, the aircraft took off from 10L, followed runway heading then turned heading 360 per instruction, but never acknowledged the turn.  Per your diagram I guess it would have been east/north departure?  I confirmed from LiveATC that it tuned to something listens on 118.275 and 125.0 (I guess it combines 2 frequencies?).  Also, the airport was very busy at that time.

Well, it's more about how busy the airspace is in the departure airspace then the actual airport.

At my airport, KCOS, tower has split frequencies for the east and west runways. 90 percent of the time both are handled by the same controller. They transmit and receive on both frequencies, but you are only listening to one.

Upvote for clarity, and because I'm former ZDV.


text:
What are interesting cases of ATC agreements between countries?
I'm doing some research about cooperative documents between nations regarding ATC; does anybody can mention interesting cases of agreements across the globe? Relations such as Swiss/Others and Hong Kong/China (during British domain) comes to my mind. Also, is there known, famous cases of events (incidents/accidents) caused by disagreement between ATC of different countries you could mention?

You will certainly want to look into Letters of Agreement.

https://en.wikipedia.org/wiki/Korean_Air_Lines_Flight_902 comes to mind

There is no problem on Swiss border as they are member of Eurocontrol and so are all their neighbours.

You could look into the situation regarding the airspace above northern Cyprus. This relates to 'disagreements', rather than 'agreements'!
Cyprus was invaded by Turkey in 1974, and since then the country has been split into two sections with a UN 'buffer-zone' between the two. The northern portion of Cyprus is not recognised as a separate country (well, not recognised by many States ...), and whenever 'Cyprus' is mentioned they are talking about the Greek half on the southern side of the island. Turkey still insists that the northern half is a separate country.
There are a few air-corridors that pass north-south (and v-v) over the country. A flight going from the north to south will be handled by Turkish ATC until they are at the FIR boundary mid way across the sea between Turkey and Northern Cyprus, whereupon they are told to contact 'Ercan Control'. Ercan is an illegal ATC station in the occupied part of the island which is not recognized by the International Civil Aviation Organization (ICAO). ICAO only recognises the legal government of Cyprus and has issued instructions to aircraft operators to obey only Nicosia ACC when in the Cyprus FIR.
In practise, flights generally ignore that instruction, and contact Nicosia ATC directly.
Heading in the opposite direction flights are told (by Nicosia ATC) to contact Turkish ATC, completely ignoring Ercan Control.
In practise, there are extremely few flights flying these routes, so the problem rarely arises. The use of Ercan Control is limited to Turkish flights heading from Turkey to northern Cyprus.
See Cyprus ATC

this is already your third account. please stop creating new ones, you will benefit from it too.

Sorry Sir, ou meant my account? This is the first time I access this site. By the way, I did set my nick name, but the site changed to this set of numbers... :/


text:
How do I modify the aircraft weight and balance after a battery replacement?
I am looking to replace my Gill battery with a Concord.  The regulations appear to allow me to do this as an owner.
However, the Concord battery weighs 2 lbs more.
I cant find it in the regs if I can legally modify the weight and balance for the airplane myself, or do I need an A&P to sign it off, possibly as a minor alteration?

You can recalculate the W&B and print up a new form but an A&P must sign off on the new W&B data.

Is the Concord battery on the approved equipment list?  Is it approved to be in your airplane due to it being PMA'd?
The point is that the battery has to be approved by the FAA and if it isn't you'll need to get it approved.  It's not a major redesign requiring form 337 but a minor alteration to it's approved type certificate.
To answer your question, you can alter your weight and balance but it must be signed off by an A&P mechanic.


text:
What is the need for twist along a wing?
Why should a wing even have a twist towards its outermost chord? How did that even come about? Why can't it be straight and aligned as it usually is?

A lot of wings have a twist to change the stall characteristics. For example on Cessna's, the wing is twisted such that a stall progresses from root to tip, leaving the ailerons effective for most of the stall and improving control-ability.

Washout or twist is a method to tailor the local incidence to achieve the ideal lift distribution over span independently of the wing's planform.
For example, if you desire to have a wing with minimum induced drag, you choose an elliptic planform. However, when stalling, this kind of wing will show a nasty roll instability because the lift coefficient is constant over span, causing one of the tips to lose lift first. To improve handling, designers learned to add chord near the tips. This by itself would create more lift at those tips than ideal and increase induced drag. So they added twist such that the local incidence decreases toward the tips. Now the wing has again the desired elliptical distribution, but has some margin at the tips to ensure that stall happens there last.
Note that washout will result in the desired lift distribution only at one angle of attack.
Washout is also necessary to adjust the lengthwise lift distribution in flying wings. Here the elevons will adjust local lift in order to trim the aircraft, and in order to avoid excessive trim deflections, washout is added so that deflections can be kept small.


text:
PPL types in the UK
Can someone explain the different PPL type for a plane? I see UK-PPL, CCA-PPL, EASA-PPL, Part-FCL.
My intentions is I will be moving to America (SF) in about 1-2 years time and want to transfer it over. Are there any difference in which one I get.
Is just as costly to learn there compared to the UK (I am in Southampton), so might as well get started.

When transferring to the United States, you'll need to work with the FAA (Federal Aviation Administration). The good news is that foreign pilot licenses can be converted to a US Private Pilot License through a process called "validation" or "conversion." EASA-PPL or UK-PPL are generally the most straightforward to convert to a US license

UK-PPL - Valid within the UK and recognized internationally, follows UK-specific aviation regulations
EASA-PPL - Standardized across European Union countries, follows common European aviation standards

I recommend starting your training in the UK, as you'll be more familiar with the local conditions and can complete your license before moving. The skills and knowledge are largely transferable, and you'll save time by not starting from scratch in the US.


text:
Is there a common maintenance issue requiring filling a hole in the tail on the A321?
I was on a flight recently (AA 705) and a maintenance worker used a forklift to go on top of the plane shortly before the captain announced that we were going to have to wait at the gate for an extra 25 minutes while they filled a hole with putty near the tail of the plane. The aircraft was an A321.
Can anyone tell me what was wrong with the plane? The pilot said it was a common problem and part of routine maintenance. I am wondering if this is a maintenance issue that an A321 expert would immediately recognize as part of routine maintenance or a problem common to these aircraft.

Airplanes are regularly delayed for minor maintenance issues, often temporary fixes that are addressed later. These are generally not publicly reported. Without more info or inside knowledge we can't really say for sure what the issue was here.

so it wouldn't appear in any logs most likely?

@dalearn it would be in the maintenance logs but they're not public information.

I'm curious why you ask.

The pilot wasn't very clear at all and downplayed the entire thing so I was wondering what was actually wrong.  (maybe a bolt fell out?)

I'm voting to reopen. Just because you cannot answer this apart from mere opinion does not mean that there is no one out there that can. There is very possibly a common issue that requires application of putty on the upper empennage of the A320 family of aircraft. If there isn't, then no harm done, this question remains without an answer.

@JonathanWalters , The pilot did say it was a common problem and part of routine maintenance which is why I asked our plane experts to see exactly how common it really was!

This question could probably be answered reasonably well by any A&P. (S)He could tell us in what scenarios the application of putty would be an acceptable fix, how short- (or long-) term that fix would be, and what might require a puttying on the backside of an A321. (Maybe it was a very new A321 and had diaper rash... :) Unfortunately, I don't think we have any A&Ps hanging around here now, but that doesn't mean we won't in the future.

@FreeMan Agreed, we need someone familiar with A321 maintenance practice. I am an A&P, but I have no familiarity with the A321 or it's maintenance practices.

@FreeMan it was a new plane from sometime this year...  :)

We have an A&P in the house! Sorry, @JonathanWalters, I'm pretty sure I knew that but totally forgot. Do you have any general insights into slathering putty on the rear of a plane? It might be the best we can get... (Frankly, I've got an image of spackling in my head that just won't go away - and that doesn't instill confidence. The fact that OP is still here to ask the question indicates that spackling isn't what was used, but still...)

@FreeMan I'm not sure if it was putty or some other type of sealant, figuring out what it was and why it needed to be done was part of my motivation for asking here!

One would hope that regular maintenance would plug all holes in tails of all planes.

While maintenance records exist at the airline, they're not something that can be accessed by the general public over the internet.  And without those records, it would be guessing to answer what exactly "filled a hole with putty near the tail of the plane" was addressing.
It sounds like the delay might have been to allow the putty to cure, but again, without knowing the exact details, that's as much a guess as anything.
If you know somebody who works in the maintenance department at AA, they might be able to get the info, but again that's not the sort of thing that airlines like being released routinely.
This question may well be closed as being essentially unanswerable, and that's probably the right "answer" in this case.  Without far more specific knowledge, it's not something one can determine from the facts given, even with the World Wide Web available for further research.

"If you know somebody who works in the maintenance department at AA, they might be able to get the info" – But they'll probably not be allowed to tell you anyway.


text:
Why does this chart seem to show airports with magnetic variation up to 90 degrees?
I am trying to understand some plots of the earth's magnetic variation (declination) by trying to correlate what I see in a figure like the one below with the variation reported for some airports. I'm particularly interested in those locations where the figure seems to show an extreme variation of up to 90 degrees. But when I look up nearby airports, I don't see any variations values outside of 22 degrees.
Some locations I have checked are

SOG, Sorong, Indonesia (1.47 degrees)
BWN, Brunei (0.13 degrees)
CMB, Bandaranaike International Airport , Sri Lanka (2.15 degrees)
REC, Recife, Brazil (22.12 degrees)
IVA, Ambanja, Madagascar (9.8 degrees)

Am I misreading the figure? What are some airports with very large magnetic variations? I'd like to compare them with this figure and try to understand what I am missing.

Source: Wikipedia

Those lines are named isogonal lines, meaning lines of equal magnetic declination (from iso=same like in isotherme, and gone=angle, like in goniometer).

I think that gone=angle would be recognized by more people from trigonometry, the measurement of bodies with three angles.

How do you think "the figure seems to show an extreme variation of up to 90 degrees"?
I don't think you're understanding that diagram correctly.
There is nothing close to a variation of 90° anywhere that is populated.
The angle of the curve has nothing to do with how strong the variation is.
All points on a single curve share the same variation.  If you find a 10° variation on a curve, and trace that curve a thousand miles away, that point will also have a 10° variation.
Look at about Washington DC, with -10° variation.  Follow that curve all the way down to the middle of South America.  That point also has -10° variation.
Look near Denver, CO, where it is +10°.  Follow that curve out to near eastern Hawaii.  The curve there is almost due East-and-West, but that doesn't matter.  That area also has +10° variation, because it is on the same curve.
The bold curves are at 10°, 20°, 30° etc variation.
The red curves have a positive (east) variation.
The blue curves have a negative (west) variation.
All of this information is in the legend at the bottom of the map.

Actually, down by the bottom right of the chart there are some areas of 90 degrees....  (Everything else is right though!)

I had a completely different idea that this represented the field line projected onto the map. Thanks for clearing that up.

@Lnafziger :  Any active cities or airports there?   I didn't think so. :)

@abelenky No, however before your edit (made 30 seconds ago), you said "There is nothing close to a variation of 90° anywhere." which was patently false, and even misleading....  :)  For that matter, it actually clouds the issue of how to read the chart and would probably be better stated as "There are areas of 90 degree variation close to the poles, however not in the areas in which you are looking..."

@abelenky look carefully and you can find an active airfield there. It is extreme case though.

Better to just remove the line about not having 90 degrees variation or explain that it's only possible near the poles (which I presume would be easy to intuitively understand) because there are people who are stationed very near both poles

Take a closer look at the map. The contour lines show the places where the magnetic deviation is the same. The green lines specifically show where the deviation is zero, so in Indonesia the deviation will be close to zero. The green line runs roughly East-West, but that's indication that there is a region roughly East-West where the declination is zero.
The more extreme deviations occur close to the magnetic poles which are, in global terms, close to the geographic poles, and there aren't many airports in those regions.
I live in Wellington, New Zealand. Take a look at the southern magnetic pole and find the nearest contour line that runs through the lower North Island - it's 22deg East, which is what I set my compass to.
Now look below that on the map to find the 90deg contour. It loops around mostly within the confines of Antarctica.
That close to the magnetic pole strange things happen. At McMurdo Station the declination is 144deg (not sure whether that's East or West but it doesn't matter for this). Before adjustment their compasses actually point South!
More info on McMurdo Station here

The lines in your plot indicate paths along which the magnetic deviation is constant.
If one plots the direction of north as displayed by a magnetic compass, one gets the blue arrows in the following image:

I've also marked the regions where the direction of magnetic north deviates less than +/-0.1° from the geographic north in green, and you can clearly see the similarity to the green lines in your plot.
In addition, a deviation of more than +/-45° is marked red, and +/-90° cyan.
If you are looking for larger airports with large deviations, check for example Greenland,  Canada, South Africa and New Zealand.
Extreme cases could also be found on Antarctica, though they don't have so much traffic...
Just as reference: I used what I still have on disk from this answer.

In fact, magnetic variation (often called declination) can be as much as 180°. If you walk from the axial south pole towards the magnetic south pole, or from axial north to magnetic north, your compass will be backwards.

Of course, the real magnetic field is rather "imperfect", as your diagram shows, so standing exactly between the two south poles might not give you exactly 180° declination. But anywhere along the line on your diagram right between the red and blue areas (your diagram shows it as the final red line) is at 180° declination.
You can count the bold lines to confirm this on your diagram.


text:
Do any civilian aircraft use AESA or PESA for weather radar?
Are there any weather radars in civilian aircraft that uses active or passive array techniques? I know that the most of the radar used in the civilian aircraft use doppler effect and latest military aircraft use AESA radars.

Related: How does a PESA radar work? PESA/AESA can be used in Doppler detection, as @foot answered. PESA/AESA is related to the type of antenna used by any radar system. Doppler and ESA can work in many ways. A good example is the Doppler VOR, where the Doppler shift is created by the VOR station antenna (which is a PESA).

Doppler and Electronically Scanned Arrays are different types of technology. Doppler is a method of using radar to determine relative velocity of a radar return. An ESA is a type of radar transceiver. Doppler is useful for detecting precipitation and would probably still be useful on an ESA radar.
Probably the main reason that weather radar has not widely gone to ESA is cost. Especially AESA (Active Electronically Scanned Array) is very expensive. In a military environment, the extra functionality makes up for the high cost. But current weather radar on aircraft works "well enough". The technology has been in use a long time and is well-understood. Advances in signal processing have brought more and more capabilities. There need to be large benefits to justify the expense of developing and certifying a new architecture.
One benefit of ESA radar can be the lack of moving parts. However, a single array would have a limited field of view, where current radars can typically see 180 degrees in front of the aircraft. They could add mechanical steering, but that would be even more expensive to steer something heavier and more delicate than the typical slotted antennas used today.
There's also the issue of military usage. Although many countries have developed ESA radars, there may be some government resistance to exporting that kind of technology.
Although current technology still struggles to detect hazards that lack precipitation such as clear air turbulence, the industry is looking to other technologies such as LIDAR to fill this gap.

Is there an performance difference between doppler and PESA/AESA when it comes to detecting precipitation and other weather phenomenon ?

You missed @fooot's point. ESA is a method of steering the radar beam electronically. It is an alternative to mechanically moving the antenna to steer the beam. It doesn't change how the radar return signal is processed. Doppler is a method of signal processing that looks for the small frequency shifts of the returns due to the motion of the targets (precipitation in the case of WxR). It's a valuable tool in the WxR as the motion can indicate turbulence and wind shear. But it's a totally separate function from the antenna design.


text:
Providing flight instruction on a "N" aircraft towards an EASA license in Europe
I need some help getting clarification on the following:
I hold a Dutch EASA CPL licence with instrument rating and FI privileges. I also hold FAA PPL with Instrument rating based my foreign (Dutch) Licence. I realise that FAR 61.3 allows me to excersise the privileges of my Dutch License on "N" aircraft within the boundries of the Netherlands.
If I were to be training a student towards an EASA Instrument rating, on an "N" registered aircraft and would venture into, say, Germany. Would this be permitted by FAA regulations (never mind the european rules).
I am in doubt since I would be pilot in command with appropriate FAA privileges (PPL(A) with IR). And although I am providing training, I am not providing training towards an FAA rating (FAR 61.3.d.2) so I would not be acting as an instructor in that sense.
Any insights are highly appreciated.

It's legal. Refer to 14 CFR § 61.3(d)(3)(v):

A flight instructor certificate issued under this part is not necessary [...] Under paragraph (d)(2)(iii) of this section, if the training is given by an authorized flight instructor under § 61.41 of this part.

Then refer to § 61.41(a)(2):

A person may credit flight training toward the requirements of a pilot certificate or rating issued under this part, if that person received the training from [...] A flight instructor who is authorized to give such training by the licensing authority of a foreign contracting State to the Convention on International Civil Aviation, and the flight training is given outside the United States.

Assuming you are authorized to give flight instruction under European rules, you can do it in a US civil aircraft. You just can't give any FAA endorsements besides that training was given. So no endorsing a student to take a practical or for solo flight, for instance. But you can endorse their logbook to show that training was given, and they can use that towards an FAA certificate or rating if they like.


text:
The evolution of airline safety videos - is it going in the wrong direction?
When the very first airline safety videos were introduced, they were very straight-forward, explaining the features of the aircraft without any extra drama / cultural element infused in them.
Over the years, airlines realised that passengers were not paying attention to the safety videos and decided to change the safety videos. That change was to infuse an element of culture of the country which the airline represents (in the case of a flag carrier) / it hails from. Some changes were also to infuse entertainment.

Air New Zealand first started this trend (or so it is said to have started this trend) with the 'Lord of the Rings - The Hobbit' safety video. Many other airlines followed suit - the most recent being Air India with its video based on Indian classical dances. Singapore Airlines (sights of Singapore), Qatar Airways (football theme), British Airways (that comedy) are many other examples.
But there are lot of concerns surrounding this. A study has shown that straightforward safety videos are retained more than their more entertaining counterparts. Also, there are risks that the passengers might 'lose the details in the distractions'.
As quoted in the study:

Tehrani and Molesworth (2015) contend that there are added advantages to improving the quality of the safety briefing on aircraft. Apart from the obvious improvements in knowledge, providing a briefing that carefully incorporates humour can positively impact on passengers' mood, which in turn can translate into improved performance (i.e., during an emergency evacuation). However, Smith (1993) offers a word of caution when using humour in advertising. He contends that humour can disrupt the systematic processing of target/advertising material/information, resulting in the target message being lost to the humorous message. Chan (2011) found that indeed humour can disrupt the processing of the target message in a study with 253 university student and five different advertisements (two humorous, two non-humorous and one neutral). Moreover, while the humorous advertisements were more successful in gaining the attention of the students, they were found to cause more disruptions during the processing of the message.

Are we going in the right direction with regard to this? Please give either yes / no + a justification for the same.

Citations:
Tehrani, Morteza & Molesworth, Brett. (2015). Pre-flight safety briefings, mood and information retention. Applied Ergonomics. https://doi.org/10.1016/j.apergo.2015.06.015

If that study actually exists it will not be hard to find... a very good question though!

I suspect it is this: https://www.researchgate.net/publication/279312801_Pre-flight_safety_briefings_mood_and_information_retention

Yes @Jamiec! Thank you! I was hunting for this. I couldn't find it anywhere else once I saw it (I guess I should be blaming Google algorithms).

I've requested the full text and will craft an answer if I get it

Obviously, some people think this is going in the right direction, others may consider it going in the wrong direction. As written, this is an opinion based question.